The superconducting coil has been put to practical use in various fields as a means of generating high magnetic fields. On the other hand, the practical application of superconducting coils to A.C. devices such as transformers and reactors has witnessed little progress due to the phenomenon of losses incurred by superconducting conductors in the presence of AC.
However, since the recent development of a superconducting conductor having a small loss of AC by the thinning of superconducting stranded wires, a progress has been made in the researches for its application to transformers and other A.C. devices, and various proposals have been made on the structure of superconducting coils made thereof.
As superconducting conductors for this case, a superconducting wire made of a metal superconductor that remains in a superconducting state at a very low temperature of 4K at which liquid helium evaporates is mainly used as a practical superconducting material. Recently, however, efforts are being made to develop superconducting coils based on an oxide superconductor. This oxide superconductor is also called “a high-temperature superconductor.” The use of this high temperature superconductor is more advantageous than the use of metallic superconductors in that the operating cost is low (see patent references 1-4 indicated below).
By the way, when a plurality of conductors are used in parallel for example in a transformer or other A.C. devices in which current changes rapidly, conductors are transposed. The relative positions of a plurality of conductors are changed to reduce the interlinkage magnetic flux between the respective conductors, or reduce induced voltage resulting therefrom and thereby make the current distribution for the respective conductors uniform.
The differences in induced voltage between respective parallel conductors resulting from the magnetic flux generated by current induces circulating current. In the case of ordinary conductors such as copper or aluminum, however, impedance consists mainly of resistance component and the circulating current has a phase deviating by approximately 90° in relation to the load current. For this reason, even if a 30% circulating current is generated, the current flowing in a conductor is the vector sum of 100% of the load current and a 30% circulating current having a phase difference of 90° thereto, and therefore, the absolute value thereof which is the square root of the sum of respective squares amounts to approximately 105%. Thus, the increase in the value of current is small for the circulating current.
When a superconducting wire is used as a conductor, on the other hand, as resistance is practically zero in the superconducting state, impedance that determines circulating current is mostly determined by inductance. Therefore, the circulating current takes the same phase as current, and if the circulating current is 30%, this circulating current is added to the current and as a result a 130% current flows in the superconductor. When this current value reaches the critical current level, the loss of AC increases or drift increases.
In such a coil consisting of superconducting wires, it is very important to control the circulating current. Although it is possible to contain circulating current in a superconducting conductor by changing the relative position of conductors, in the case of oxide superconducting wires which are by nature weaker than alloy superconductors to the bending force, there is an allowable bending radius for displaying their capacity, and it is necessary to pay the maximum attention to the work of transposition. Therefore, the more numerous is the number of parallel conductors is, in other words, the more numerous is the number of transposing parts, it takes longer time to do the work and the whole project becomes more costly. And even if sufficient attention is paid to the transposition part, due to the deflection of superconducting wires, it is unavoidable that such parts would be unstable, and such unstable parts become more numerous as the number of transposition parts increases.
In a superconducting transformer in which there are only a limited number of coil layers, where there are rooms between layers and the coil diameter is large, the countermeasures taken against the unstable parts are easy, and the conventional transposition method is enough. However, in the case of coils for storage of energy or those for magnetic field application, due to a large number of coil layers and the requirement for keeping the layers in close contact, the space for taking countermeasures against the unstable parts will be limited. Therefore, the impacts of the countermeasures against the unstable parts may affect other upper and lower layers or contiguous superconducting wires. And not only there is a risk of being unable to meet the required specifications but also the problem of being unable to keep stable operation.
The structure of a superconducting coil designed to solve the problems described above, to reduce the number of transposition parts as unstable parts while containing circulating current, and to reduce the costs by simplifying the transposition work is disclosed for example in Patent Reference 1.
The summary of the invention described in Patent Reference 1 is as follows. Specifically, “in a superconducting coil in which a plurality of superconducting wires are arranged in parallel and wound, it is possible to reduce the number of transposition parts, contain the circulating current and at the same time reduce the unstable parts by adopting a structure in which the relative positions are changed only at the ends of coil, and in addition by making the number of coil layers an integral multiple of 4 times the number of superconducting wires arranged in parallel (4 times the number of wires). As a result, the work and time for transposition is reduced resulting not only in lower costs, but also fewer unstable parts and thus enabling to contain circulating current. Therefore, it is possible to obtain an advantage of being able to excite and demagnetize at a high speed and stably”.
FIG. 10 is an example of the transposition structure of a superconducting coil described in FIG. 1 of Patent Reference 1. In FIG. 10, for winding three superconducting wires 3a superposed in the radial direction of the coil by winding in the direction of bobbin 1a-bobbin 1b, at the start of the coil on the 1a side of bobbin, the superconducting wires 3a are wound for multiple layers and from the internal diameter of the coil, for example, in the order of (A1, A2, and A3) not shown, and at the transposition part 2 at the end of the coil, at first (A3) is bent at the following turn, and the transposition work is carried out on (A2, and A1) in the same manner, so that at the end of the coil on the 1b side of the bobbin, the coil will be arranged for example in the order of (A3, A2, and A1). By making the arrangement described above, the number of transposition parts and bending of coil will be reduced in comparison with the prior transposition structure described in FIG. 4 of Patent Reference 1 and the work will be considerably simplified thereby.
Regarding an example of the structure mentioned above on a number of coil layers equal to an integral multiple of four times the number of superconducting wires arranged in parallel (4 times the number of wires), the description is omitted here (for the details, see Patent Reference 1.)
In the superconducting coil described above, on the other hand, a structure in which the generation of heat subsequent to the A.C. loss is effectively removed and a stable operation is assured without causing any normal conduction transition is required. As a structure preferable from this viewpoint, Patent Reference 2 discloses “a superconducting coil having a heat transmission cooling plate made of a material with high thermal conductivity between layers of superconducting coils wound on the peripheral surface of a cylindrical bobbin made of an electric isolating material constituting cylindrical layers.”
And as a preferable production method of the oxide superconducting wire (a high temperature superconducting wire) of a high productivity described above, a possible method is, for example, that of forming a film of oxide superconducting material on a flexible tape substrate. And production methods based on the vapor phase deposition method such as laser ablation method, CVD method, etc. are now being developed. Oxide superconducting wires made by forming an oxide superconducting film on the tape substrate as described above have an exposed superconducting film on the outermost layer, and no stabilization treatment has been applied on the surface of the exposed side. As a result, when a relatively strong current is given to such an oxide superconducting wire, the superconducting film transits locally from the superconducting state to the normal conducting state due to the local generation of heat, resulting in an unstable transmission of current.
For the purpose of solving the problems mentioned above, and providing an oxide superconductor having a high critical current value, capable of transmitting current with stability and whose stability does not deteriorate even after an extended period of storage and the method of producing the same, the Patent Reference 3 discloses a following tape-shaped superconducting wire.
Specifically, “a superconducting wire comprises of an intermediate layer formed on a flexible tape substrate, an oxide superconducting film formed on the intermediate layer, and a gold or silver film (a metal normal conduction layer) 0.5 μm or more thick formed on the oxide superconducting film.” And example of embodiment described in Patent Reference 3 reads as follows. “On “Hastelloy” tape serving as the substrate, an yttria stabilized zirconia layer or magnesium oxide layer is formed as an intermediate layer. On top of this layer, Y—Ba—Cu—O oxide superconducting film is formed. And on this layer, a gold or silver coating film is formed.”
And for the purpose of effectively dissipating the heat generated by AC loss and for improving thermal stability by forming a normal conductance metallic layer, Patent Reference 4 discloses the method of producing superconducting wires in the form of a tape having the following structure.
The Japanese patent application laid open describes as follows: “A method of producing high temperature superconducting wires wherein said high temperature superconducting film of a tape-shaped material made by coating a high temperature superconducting film on the substrate surface is irradiated on the longitudinal direction by one or more long-wave laser beam arranged horizontally by intervals to deprive its superconductivity (change into normal conductor) the irradiated part, and at the same time the width of the superconducting parts located between said non-superconducting parts is controlled by the non-irradiation of long wavelength laser beam by choosing the beam diameter and the distance between said plurality of long-wave laser beams.”
Patent Reference 1: Japanese Patent Application Laid Open 11-273935 (p. 2-4, FIGS. 1-4)
Patent Reference 2: Japanese Patent Application Laid Open 11-135318 (p. 2-4, FIG. 3)
Patent Reference 3: Japanese Patent Application Laid Open 7-37444 (p. 2-7, FIG. 1)
Patent Reference 4: Japanese Patent Application Laid Open 3-222212 (p. 1-2, FIG. 3)
When mass-produced tape-shaped superconducting wires like the ones described in Patent References 3 and 4 mentioned above are used in an A.C. device, the A.C. loss that develop in the superconducting wires will be, due to the form anisotropy of flat tapes, dominated by those in the perpendicular magnetic field acting in the perpendicular direction upon the flat surface of the tape. This is because demagnetization that accompanies changes in the magnetic field, in other words, the magnetic momentum m for canceling the magnetic field is the product of multiplying the shielding current i by the average distance d of the shielding current, and therefore in the case of the flat tape shape, the average distance d of the flat surface is far greater than that in the thickness direction of the tape, and the magnetic momentum m will be far greater in the perpendicular magnetic field acting upon the flat surface.
Therefore, in order to reduce A.C. loss, how the perpendicular magnetic field loss can be reduced, or how the shielding current i and the average distance d of the shielding current on a flat surface can be reduced will be a problem. From this viewpoint, the structural separation of the superconducting film part of the tape-shaped superconducting wires will be effective to reduce the average distance d mentioned above, However, in the case of the superconducting wires described in Patent Reference 4, the normal conducting film part and the superconducting film part are alternately formed and therefore eddy current losses develop in the normal conducting film part to amplify the losses.
When a superconducting coil is made by using mass-produced tape-shaped superconducting wires described in Patent References 3 and 4 above, it is difficult in view of the structure of the superconducting wires to change the relative positions described in Patent Reference 1, and even if such transpositions are carried out, instability resulting from the transpositions increases.
Therefore, when tape-shaped superconducting wires are used, it is preferable to adopt a structure of not causing changes in the positions, making shunt current uniform and of containing circulating current. And as for the structure of coil, it is preferable, from the viewpoint of the structure or arrangement of the superconducting coil, to adopt a structure that cancels the perpendicular interlinkage magnetic flux that acts on the superconducting wires in order to reduce A.C. loss due to shielding current. Moreover, it is preferable to adopt a structure that makes it possible to cool down the superconducting wires as uniformly as possible and to increase the current-carrying capacity thereof.
The present invention has been made in view of the points described above, and the objects of the present invention are to provide a superconducting wire capable of containing A.C. loss and a low-loss superconducting coil made from this superconducting wire having a simple structure without transposition, capable of canceling interlinkage magnetic flux due to the perpendicular magnetic field to the wire, and capable of containing the circulating current within the wire due to the perpendicular magnetic field and making shunt current uniform so that the losses may be limited.